In order to find out the reason of instant opening after closing of type CT20-Ⅲ spring operating mechanism for 220 kV circuit breaker

in this paper the faulty circuit breaker is checked at site

the operating mechanism is tested and verified

disassembled and checked.It is shown by the study result that reason of instant opening after closing of the circuit breaker is due to higher closing speed and fatigue of resetting spring of the operating mechanism

causing significant bounce increase of both closing and tripping holding latch.At the same time

the design of the closing holding and opening tripping system of the circuit breaker is unreasonable and the locking at two places shall be established simultaneously for the closing holding.Both the closing and tripping holding latches are susceptible to the closing speed

causing the instant opening after closing.Finally

the improvement measures are proposed combined with the reasons for the failure of the circuit breaker so to improve the safe and reliable operation level of the equipment.